The answer above actually gets the type union of all non-array elements of a multi-level array.
In other words
Flatten
will give you a union type of 1, 2, 'a', and 'b'
const foo: Flatten = 'b'; // OK
const bar: Flatten = 'c'; // Error: Type '"c"' is not assignable to type '1 | 2 | "a" | "b"'
Technically the inference is unnecessary there, if that's you're goal:
type Flatten = T extends Array ? Flatten : T
I don't really consider this the type of flattening an array, but `Array>` would be. And this would actually be comparable to the builtin Array.prototype.flat type signature with infinite depth (you can see the typedef for that here[1], but this is the highest level of typescript sorcery)
My solution was for flattening an array with a depth of 1 (most people using Array.prototype.flat are using this default depth I'd wager):
console.log(JSON.stringify([1,[2, [3]]].flat()));
> [1,2,[3]]
The type I provided would match those semantics:
// 'readonly' added to the 'extends' sections to work on "as const" (readonly) arrays
type FlatArr = Arg extends readonly [infer First, ...(infer Rest)] ?
First extends readonly unknown[] ?
[...First, ...FlatArr] :
[First, ...FlatArr] :
[];
const flatten = (arr: Arr): FlatArr => arr.flat() as FlatArr
const someArr = [1,[2,'a',['b', ['c']]]] as const; // const someArr: readonly [1, readonly [2, "a", readonly ["b", readonly ["c"]]]]
const someArr2 = flatten(someArr); // const someArr2: [1, 2, "a", readonly ["b", readonly ["c"]]]
[1]:
https://github.com/microsoft/TypeScript/blob/main/src/lib/es...